Chromium alloy plating



United States P n 2,927,066 (:HRoMiUM ALLOY PLATING No Drawing. Application December 30, 1955 Serial No. 556,462

11 Claims. (Cl. 204-43) This invention relates to coatings containing chromium.

More particularly, it relates to a process and to an elec trolytefor electrodepositing chromium alloys. It is frequently desirable to provide a decorative coating for metal surfaces which can be buffed to a high luster, which will retain its hardness, and which will retain its hardness, and which will protect against corrosion. Conventional chromium plate usually has a bright surfaceonly when it is deposited on a smooth bright metal, but when the conventional chromium plate is deposited on a rough,dull base metal, the plate is dull, rough, and diflicult to buff to a high luster. Also, conventional chromium plates will not retain hardness when heated and do not provide adequate protection against corrosion or high temperatures.

Suitable trivalent chromium salt baths can be used to electrodeposit chromium alloy plates with a high degree of success. In thecopending application of William H. Safranek,Serial No. 487,697, filed February ll, 1955,

there is disclosed a chromium-alloy platingbath contain-- ing trivalent chromium ions, ions of an alloying metal, and ions which increase the conductivity of the solution. The invention described in that application was particularly suitable for electroplating chromium-iron alloys having a low percentage of iron. The electroplates obtained were considerably harder after heating than unalloyed chromium.

There were several disadvantages in the invention described in application. Serial No. 487,697. To obtain best results when plating at the preferred current densities of 100 to 150 amperes per square foot; it was necessary to maintain the pH in the narrow range of 1.9 to 2.1. At a higher bath pH, the electrodeposits were of poor quality. This is believed to be due to the formation of a film of ferrous hydroxideon the cathode surface, where the pH was considerably higher. thanin the bath.

A further difficulty with the process of the prior inven-. tion was that there was some tendency for ferric ions to form, even when iron-silicon-alloy anodes or soluble chromium-iron-alloy anodes were used. The presence of ferric ions near the cathode resulted in undesirable stresses in the chromium-alloy plate. Continuous operation for long periods of time was impractical without special pre-.

cautions. A further disadvantage was that the baths of the prior invention had too poora throwing power for satisfactory plating on the surfaces of complex shaped parts. 1

a chromium-alloy plaitng bath having improved throwing power.

A further object is to electrodeposit a chromium-alloy plate which will retain its hardness after heating to elevated temperatures.

A further object is to electrodeposit a chromium-alloy Serial No. 487,697.

Patented Mar. 1, teen "ice 7 2 plate which is rustproof and relatively free from the cracking which exists in conventional chromium electrodeposits.

A further object is to electrocleposit chromium alloys by a process which is more easily reproducible than those known heretofore.

These and other objects will be apparent. from the specification and examples which follow.

According to the present invention, a chromium alloy is electroplated from an, aqueous solution containing trivalent chromiumions, ions of an alloying metal, and sulfamate ions. Additional ions may be added to improve the conductivity of the solution. Ammonium snlfamate and the alkali metal sulfamates are preferred according to this invention, although any soluble snlfamate may be used. The presence of snlfamate ions in the plating bath eliminates the difficulties encounterd in prior art chromium-alloy plating baths, and makes it possible to plate high-quality chromium-alloy plate continuously for long periods of time.

In the baths of this invention, trivalent chromium ions are provided by trivalent chromium salts or other compounds, such as chromium ammonium sulfate, chromium sulfate, lbasic chromium sulfate, chromium fiuoborate, chromium chloride, chromiumcarbonate, chromic oxide, or potassium chromium sulfate. Supplementary salts, such as ammonium sulfate, sodium sulfate, sodium fluoborate, sodium chloride, or a mixture of such salts, are added to provide ions that improve the conductivity of the electrolyte. Ions of alloying metals may be added as metal salts ofcompounds, such as metal sulfates, metal ammonium sulfates, or metal fluoborates. The alloying metals suitable according to the present-invention are the iron group metals, that is, iron, cobalt, and nickel. A suitable electric current is passed through the solution, while maintaining it at a proper temperature and pH, for a sufficient length of time todeposit a. coating of desired thickness. i

' Chromium alloy plates may be obtained from baths according to the present invention using a pH of from 1.0 to about 3.5; however, to obtain the best results, the preferred range is from about 1.4 to about 2.8. The optimum pH range is from 1.9 to 2.4 when plating at the preferred current density of 100 to 150 amperes per square foot. This compares with an optimum pH of 1.9 to 2.1 according to the process disclosed in copending application, The snlfamate additives of the present invention extend the optimum pH range.

The pH of the baths may be adjusted by adding acid or alkaline reagents, such as sulfuric acid, or ammonium hydroxide, respectively. Plating baths made up of chromium ammonium sulfate, chromium potassium sulfate, chromic sulfate, and chromium fluoborate are acid in nature and require initial additions of alkaline reagents, such as ammonium hydroxide, to obtain the proper pH. Trivalent chromium baths made up from basic chromium sulfate, chromium carbonate, and chromic oxide require acid additions to obtain the proper initial pH. Sulfuric acid is preferred. During operation of the baths, either those in which chromium ammonium sulfate or other trivalent chromium chemicals have been added, it may be necessary to add small amounts. of acid to prevent the pH from rising. Boric acid may be added to the baths as a butter reagent to stabilize the pH. For best results, the borate ion concentration should not exceed 45 grams per liter. If alkaline reagents, such as ammonium hydroxide, areadded to the bath after it has been prepared, special s a soluble sulfamate according to the presentinvention improves chromium-alloy plating in several respects. In the first place, the optimum pH range is broader, as already indicated. Sulfamate ions prevent the formation of hexavalent chromium and ferric ions in solution; regducing the stress in the electro-deposite d alloy. fur: ther advantage of sulfamate ions is that they inhibit the precipitation of a ferrous hydroxide film on the cathodes during start-up of a cell. Such films, which havebee'i previously encountered in chromium-iron alloy electroplating operation, impaired the adherence and the purity of the electroplate. v

A significant advantage in the presence of a according to the present invention is the resultant increase in throwing power of the bath. Ammonium snl; famate and the alkali metal sulfamates are particularly desirable in this regard. Throwing power. isconsider ably improved by operation at a pH of 2.1 to 2.4 .a s compared to a pI-lof 1.9 to 2.1, The presence of a sulfaniate makes operationin the pH range of 2.1 to 2.4 feasible. Ammonium sulfamate is particularly desirable as an additive because it improves the microfilling properties of the chromium alloy plate, while the alkali metal sulfamates have a slight adverse effect on microfilling. Microfilling is the power of a coating layer to fill microscopic holes and scratches on the surface of the basis material to which the coating layer is applied, so that the surface of the coated layer is smoother thanthesurface' of the basis material. H p I Theconcentration of the sulfamate may be varied over wide limits. For exampla in the case of ammonium- 7 assulfamate ions; The amountlof sulfamate shouldbe.

kept sufficiently low to avoid any undue impairment of cathode efiiciency lthasbeen found that the amount of ammonium sulfamate should not be allowedto exceed approximately 80 grams per liter for this reason. This corresponds to about 65 grams per liter of sulfamateions. This, in general, is the maximum concentration of sulfarnate ions consistent with reasonable current efficiency. A concentration of about 60 grams per liter of ammonium sulfamate is preferred. Alkali metal sulfamate, such as sodium sulfamate and potassium sulfamate also may be used in amounts which furnish 8 to 65 grams of sulfamate ions per liter. I

Triv alent chromium salts are commercially prepared by reducing chromic, acid or sodium dichromate with an organic; reducing agent, such as alcohol, molasses or with sulfur dioxide. I ducing agent, and oxidation products thereof, since an excess of the reducing agent is used to complete thereduction. Thus, plating, solutions made up of trivalent chromium salts will contain these harmful impurities. The excess reducing agents, and oxidizing products thereof, and any other harmful impurities in. the baths can be removed by the addition of a small amount of activated carbon, preferably by adding a slurry of the finely divided powder in water. The activated carbon removes the impurities by means of absorption. The purification of the plating baths by means of activated carbon results in smoother and better plates.

As has been stated,trivalent chromium ions can b'e supplied to the plating baths as trivalent chromium salts, such as chromium ammonium sulfate, chromium sulfate, basic chromium sulfate, chromium fluoborate,

chromic chloride, chromium carbonate and chromium potassium sulfate. Other trivalent chromium compoundsv which are not salts, such as chromic oxide, also cand e;

isoperated and the chromium'salt or cohipohnd added to" ts.

gei iafit r e li a u na i9i I---. l=, ist n have been obtained with concentrations-ranging roirr'ifl t ffen The; product contains residual re-' grams per literto saturation. However, concentrations of from 28 to 45 grams per liter have been found to be the most satisfactory. In general, smaller trivalent chromium ion concentrations will result in better throwing power.

The alloying constituent of the plating bath is present as a salt of the alloyingrnetal. Suitable alloying metals are the iron group metals, that is," iron,- cobalt, and nickel. Iron is particularly suitable as an alloying metal. Iron should be presentin the plating bathas a ferrous salt, as the presence of ferric ions has a deleterious effect on the properties of the plate. Iron may be present as ferrous ammonium sulfate, or ferrou's sulfate in chromium sulfate and chrome alum'pla'tiiig baths. In plating baths where chromium fiuoborate is used as the source of trivalent chromium ions, the alloying metal preferably should be added as the fluoborate. For instance, if iron'is, the alloying element, it should be added as forrous fluoborate; however, mixed sulfate andfiuoborate' may be used. Nickel salts,- Such as nickel sulfate may be present in the bath Where nickelis the desired alloying element. Likewise, cobaltsalts, such as cobalt sul-' fatie or cobalt ammonium sulfate, may also be present.

The amount of alloying metal ions added to the bath is dependent upon the composition of the alloy plate desired. Generally, the alloy platesaccording to the present invention contain not more than about 25 percent by weight of the iron group metal and the balance chromium, in other words, in all cases the amount of the iron group metal is less than the amount of chromium by weight. For example, a chromium alloy plate containing 15 percent iron and percent chromiurnis a particularlyjesirable plate by virtue of its low stress coupled vith high hardness after heating; Another alloy plate which may be advantageously plated is an alloy Containing- 974' percent chromium and ,6 percent iron. Tojobtain the preferred alloy plates. of the present'inyentionythe concentration of ferrous ions is maintained between 0.6 and 2.5 grams per liter. The optimum range of cobalt ions for producing chromium-cobalt plates is also 0.6 to 2.5 grams per liter. The optimum concentrationof nickel ions for chromium-nickel alloy plates is somewhat lower, rangingfrom 0.1 to 1.0 grams per liter.

Various ternary or even quaternary combinations of these metals can be electrodeposited in the form of adherent' hardalloy plates; For example, a chromiumiron-ni'ckel alloy or. a chromium-iron cobalt' alloy can be electrodepositedrby means of the electroplating baths of-thisinvention; Nickel or cobalt salts or both nickel and cobalt salts are. added to the bath in addition to iron salts and trivalent chromium compoundsto obtain ternary or quaternary alloy plates.

Alkali-metal or ammonium compounds in addition to the sulfarnate are added to the'bath to maintain good throwing power and to improve the conductivity of the bath; Ammonium sulfate, sodium sulfate, and sodium fiuoboratei have all proved to be satisfactory when used for this purpose, but other alkali metal compounds, such as-sodium chloride, ammonium chloride, and potassium sulfate may be used" as well. Ammonium compounds have been-foundto bathe most desirable. Double salts ofa plating metal which contain ammonium or alkali metal ions maybe added-to the bath, either in addition to or instead ofthe simple ammoniumand alkali metal salts. Examples of double salts which may be added to the plating bath are chromium ammonium sulfate; chromium potassium sulfate, and ferrous ammonium sulet t la Preier e' qreretie ts mium m s it m .5 fate is'the source of trivalent chromium ions and one of the sources of ammonium ions. Additional ammoniuni en are added as ammonium", sulfate. Ammonium.

ions" also may be introduced into the bath by a e tweet hrt xid Entit eme t l t. they 3" best're s'u ts; the 'tot'al concentration of tuiiz'ironmm plus alkali metal ions should bewithin the range of from to 150 grams per liter.

In baths where sulfate salts are employed,the total sulfate ion concentration should be within the range of from 80 to 600 grams per liter, and in fluoborate baths the total fiuoborate ion concentration should not exceed 500 grams per liter. No sulfate is necessary in fluoborate baths. Chloride salts, such as sodium chloride, may be used to partially replace sulfate or fluoborate salts; however, the chloride ion concentration should not exceed 40 grams per liter.

The preferred amounts of the constituents of plating baths according to this invention are. indicated 'in Table I below:

1 This minimum amount of 80 grams per liter applies to baths which do not contain fluoborate. Sulfate may be omitted altogether in bath containing fluoborate. V

1 At least one alloying metal should be present. These ranges apply when only one alloying metal is used. The ranges for each metal may be lower when two or three alloying metals are used.

In addition to the ingredients listed in the table, other ingredients which do not harm the properties of the electroplates may be present.

Satisfactory plating has been obtained by using cathode current densities from 75 to 500 amperes per square foot. The highest quality electroplates are obtained when the current density is in the lower portion of this range. For instance, the best electroplates from baths containing 10 gramsof ammonium sulfamate per liter are obtained at current densities of 75 to 250 amperes per square foot. As the sulfamate concentration is increased, the optimum current density range becomes slightly narrower. A current density of from 75 to 230 amperes per square foot is preferred for plating from baths containing 60 grams of ammonium sulfamate per liter. For convenience and high efficiency, current densities in the range of 100 to 150 amperes per square foot are most desirable regardless of sulfamate concentration. The proper current densities also depend upon the concentration of trivalent chromium salt and other bath additions used. Other factors affecting the choice of the proper current density include the size of the plating tank, the shape and contour of the part, and the time required to produce a given thickness. For example, it has been found that the chromium alloy can be electrodeposited at approximately 0.00125 inch per hour at 100 amperes per square foot. The plating rate is, of course, faster at higher current densities.

In the operation of the plating baths of this invention, the temperatures ofthe solutions should be, preferably, within therange of from 105 F. to 145 F. For best results the temperature of any individual bath being operated at an unchanging current density should not vary more than 5 degrees from the operating temperature selected. For plating at 100 to 150 amperes per square foot, the preferred temperature is from 110 to 120 F.

It is commonly known in commercial electroplating processes that, as the metal or metals are electrodeposited out of the bath onto the cathode, they must be replaced at the same rate. In the case of alloy electrodeposition,

r '5 the metals must be replaced not only at the same rate but also substantially in the same ratio. When the plating baths of this invention are used, this replacement may be made when using either insoluble or soluble anodes, as will hereinafter be described.

When insoluble anodes are used, the bath is not replenished in chromium or iron. As a result of continuous operation, the concentrations of the chromium and alloying elements will diminish proportionally with the time of electrodeposition. These metals are constantly replaced by additions of metal compounds, such as the trivalent chromium salts or compounds originally used, and their alloying salts or soluble metal oxides or hydrates. The use of metal hydrates for replenishment has proved to be very satisfactory in the sulfate salt baths, in that excess sulfates do not build up in the bath." Due to the fact that chromium hydroxide is only slightly soluble in weak acid solutions, such as the plating baths, a special form of chromium hydrate, prepared as described in U.S. Patent 2,436,509, should be used. Chromium hydrate, as prepared by this method is both pure and soluble.

'When it is desired to operate the plating bath continuously and for long periods of time, the most satisfactory results are obtained by using chromium-alloy anodes. The use of soluble chromium-alloy anodes simplifies the plating process in that the need to replenish the bath with trivalent chromium salts or compounds is reduced or, upon proper balancing of the bath and plating operation, may be completely eliminated. v r

This invention will now be described further with respect to specific examples. In all of the examples which follow, glass electroplating tanks and platinum anodes were used.

Example I Chromium-iron deposits were obtained from a bath having the following composition:

Chromium ammonium sulfate, Grams per liter Cr(NH (SO -12H O 350 Ammonium sulfate, (NH SO Ferrous ammonium sulfate,

Fe(NH (SO -6H O 6.0 Ammonium sulfamate, NH SO NH 30 The pH was 1.85. The cathode current density was 100 amperes per square. foot, on the average, and the temperature was maintained at plus or minus 4 F. The cathode efficiency was 23.0 percent.

Example 2 The electroplating bath had the same composition as that described in Example 1, except that 60 grams per liter of ammonium sulfamate were present. The pH was 1.9, the temperature Was 115 plus or minus 4 F., and an average cathode current density of 100 amperes per square foot was used. The cathode cfiiciency was 19.7 percent. While the average cathode current density was 100 amperes per square foot, the cathode was so placed square foot. Satisfactory deposits were obtained at from 80 to 230 amperes per square foot.

Example 3 A chromium alloy was plated from a bath initially havingthe following composition: Chromium ammoh'iiii'ifsulfate,

Anastasiamainstream-H (29% milliliters er liter 0.375

The pH-of the bath-was adjusted to la9'by ammonium hydroxide; The temperature was maintained at 115 F., and-the average cathode current density was 100 amperes per square foot; Platinumanodes' surrounded by porous cups were used.- The total volume of the bath was 36 liters.

,About 150 grams of activated carbonwas slurried in 500 milliliters of water andstirred into the plating bath prior to plating. Stirring was continued for one hour, during: which time the temperature was maintained at 160 F. After filtering the carbon, chromium-iron alloy plates up to 0.025 inch thick were plated on steel and copper panels. The bath was continued in operation for four months. Appropriate additions of chromium am monium sulfate andferrous ammonium'sulfate were added to replace the metals deposited on the cathodes, keeping the ranges between 32' to 40 grams per liter of chromium, and 08 to 0.9 grams per liter of iron. Sulfuric acid and ammonium hydroxide wereaddedwhen required to keep thepH betweer'ij 119 and 2.0. Alloy electroplates containing about 90 percent by Weight of chromium and 10 percent iron were obtained.

Example 4 A chromium-iron alloy was plated from a bath of the following composition.

Chromium ammonium sulfate, Grams per liter Cr(NH (SO 121-1 0 310 Ammonium sulfate, (NHQ SO 100 Ferrous ammonium sulfate,

Fe(NH (5063 -63 0 6.5 Ammonium sulfamate, NI-I SO NH 60 The pH was maintained at 1.95, and the temperature of the bath was 115 F. Plating was continued for 1% hours" at an average cathode current density of 100 amperes per square foot. iron alloy .0037 inch thick and hav'ing a composition of about 90 percent chromium and 10 percent iron was formed. Periodic spot-checks were made to determine whether or not any ferric iron was present in the plating bath, and none was found at any time.

A water slurry of activated carbon, similar to that described in the previous example was added to the plating bath prior to plating.

. Example 5 A chromium-iron alioyjwas plated from a bath having the-following" composition:

chromium ammonium sulfate, Grants per liter -Cantu sow-1211 0;; 41o sulfate Ferrous sulfate,

,F (N 4)2( Q4)2I H2 Ammonium sulfamate, NH SO NH 60 An electrodeposited chromium sxampr 6' A chromium-iron alloy was electrodeposited from bath having the following: composition:

Chr'oi'niu'rii' ammonium sulfate,- Grams per liter The pH of this bath was adjusted to 2.25 by the addition of ammonium hydroxide. This bath was the same bathas that used in Example 5, with chromium ammonium plate having a composition of about percent chro miumand" 10 percent iron was obtained. The thickness varied from 0.0015 tof0;002 inch. This bath had better" throwing power than the baths of- Examples 1 through 5.

i Example 7 A chromium-iron'alloy was platedfrorn a-bath having the same composition as that described in Example 6', except that the pH was adjusted to 2.4 with ammonium hydroxide. A continuous, crack-free chromium=iron alloy plate about 0.002 inch thick, containing about 90 percent chromium and 10' percent'iron was-obtained. Thatamwing power of this b'ath was superior totha't' describedin" Example 6.

It is understood that various modifications may be made without departing from the principles of this invention. It is therefore understood that no limitations onthe scope of this invention are imposed, except by the appended claims.

What is claimed is:

l. The method of elect'rodepositing a chromium-iron alloy plate which comprises electrolyzing an aqueous bath consisting essentially of from 20 to 75 grams per liter of trivalent chromium ions, 10 to gr'arhs'p'e'r liter of ions" chosenfrom the group consisting of the alkali metal and ammonium ions, 0.6" to 2.5 gramsper liter of ferrousions, and 8 to 65 grams perliter ofsuliama'teion's.

2'. A composition of matter for electrodepositing' a chromium-iron alloy plate which comprises an 'aque'c'ius bath consisting essentially of from 20m 75 grams per liter of trivalent chromium'ions', 10 to 150 grains per liter of ions chosenfroin the group consisting of alkali metal and'arn'moniuin" ions, 0.6 to 2 .5 grams per liter offerrous ions, and 8 to 65 grams'per liter of 's'ulfarnate' ions.

3. The method of electrodepositing a chromium-alloy plate which comprises electroly'zing an aqueous bath consisting' essentially of from 20 to 75 grams per liter of trivalent chromium ions, from 10 to 150 grams per liter of ions chosen from the group consisting of alkali metal and ammonium ions, ions in an amount substantially less than said trivalent-[chromium ions of at least one alloying metal selected from the group consisting" of iron, nickel, and cobalt, and from 8 to 65 grams per liter of" sulfarnate ions.

4. The method of electro'depos'iting" a chromium-alloy plate which comprises electrolyzing an aqueous bath con-' sisting essentially of from 20 to 75 grams per liter of trivalent chromium ions, from it) to 150 grams per liter of ions chosen from the group consisting of alkali metal alloy plate which comprises electrolyzing an aqueous bath consisting essentially of from 20 to 75 grams per liter of trivalent chromium ions, from 10 to 150 grams per liter of ions chosen from the group consisting of alkali metal and ammonium ions, ions of at least one alloying metal selected from the group consisting of from 0.6 to 2.5 grams per liter of ferrous ions, from 0.1 to 1.0 grams per liter of nickel ions, from 0.6 to 2.5 grams per liter of cobalt ions, and from 8 to 65 grams per liter of sulfamate ions.

6. The method of electrodepositing a chromium-nickelalloy plate which comprises electrolyzing an aqueous bath consisting essentially of from to 75 grams per liter of trivalent chromium ions, from 10 to 150 grams per liter of ions chosen from the group consisting of alkali metal and ammonium ions, from 0.1 to 1.0 grams per liter of nickel ions, and from 8 to 65 grams per liter of sulfamate ions.

7. The method of electrodepositing a chromium-cobaltalloy plate which comprises electrolyzing an aqueous bath consisting essentially of from 20 to 75 grams per liter of trivalent chromium ions, from 10 to 150 grams per liter of ions chosen from the group consisting of alkali metal and ammonium ions, from 0.6 to 2.5 grams per liter of cobalt ions, and from 8 to 65 grams per liter of sulfamate ions.

8. A composition of matter for electrodepositing a chromium-alloy plate which comprises an aqueous bath consisting essentially of from 20 to 75 grams per liter of trivalent chromium ions, from 10 to 150 grams per liter of ions chosen from the group consisting of alkali metal and ammonium ions, ions in an amount substantially less than said trivalent chromium ions of at least one alloying metal selected from the group consisting of iron, nickel, and cobalt, and from 8 to grams per liter of sulfamate ions.

9. A composition of matter for electrodepositing a chromium-iron-alloy plate which comprises an aqueous bath consisting essentially of from 20 to grams per liter of trivalent chromium ions, from 10 to grams per liter of ions chosen from the group consisting of alkali metal and ammonium ions, ions of at least one alloying metal selected from the group consisting of 0.6 to 2.5 grams per liter of ferrous ions, from 0.1 to 1.0 grams per liter of nickel ions, from 0.6 to 2.5 grams per liter of cobalt ions, and from 8 to 65 grams per liter of sulfamate ions.

10. A composition of matter for' electrodepositing a chromium-nickel-alloy plate which comprises an aqueous bath containing from 20 to 75 grams per liter of trivalent chromium ions, from 10 to 150 grams per liter of ions chosen from the group consisting of alkali metal and ammonium ions, from 0.1 to 1.0 grams per liter of nickel ions, and from 8 to 65 grams per liter of sulfamate ions.

11. A composition of matter for electrodepositing a chromium-cobalt-alloy plate which comprises an aqueous bath containing from 20 to 75 grams per liter of trivalent chromium ions, from 10 to 150 grams per liter of ions chosen from the group consisting of alkali metal and ammonium ions, from 0.6 to 2.5 grams per liter of cobalt ions, and from 8 to 65 grams per liter of sulfamate ions.

References Cited in the file of this patent UNITED STATES PATENTS 2,458,839 Dyer et al Apr. 19, 1944 2,489,523 Clifton Nov. 29, 1949 2,693,444 Snavely Nov. 2, 1954 

1. THE METHOD OF ELECTRODEPOSITING A CHROMINUM-IRON ALLOY PLATE WHICH COMPRISES ELECTROLYZING AN AQUEOUS BATH CONSISTING ESSENTIALLY OF FROM 20 TO 75 GRAMS PER LITER OF TRIVALENT CHROMIUM IONS, 10 TO 150 GRAMS PER LITER OF IONS CHOSEN FROM THE GROUP CONSISTING OF THE ALKALI METAL AND AMMONIUM IONS, 0.6 TO 2.5 GRAMS PER LITER OF FERROUS IONS, AND 8 TO 65 GRAMS PER LITER OF SULFAMATE IONS. 